We are using a flow cell in which the partial pressure of oxygen can be varied rapidly in order to study sickling under conditions that more resemble physiological conditions than have been used in the past. Since there is a broad distribution in the cellular hemoglobin concentration, many cells need to be analyzed simultaneously. The change in shape of the cells in the flow cell is observed under a microscope. A video camera transmits the picture to a computer, where the fraction of cells that have sickled can be automatically determined using image analysis algorithms.